Electrical power networks are most often operated at a nominal and fixed voltage and frequency. The connection of certain types of equipment, such as for example generators, that output power with variable voltage and/or frequency may be accomplished using electronic frequency converters also known as power converters. There are different methods to generate gate switching signals to operate power converters, of which one known method is DTC (Direct torque control) and another well known method is PWM (Pulse width modulated). Power converters may be arranged to control a variable power output and convert it into an acceptable power input to a power network with fixed nominal characteristics. For example wind generators tend to have an electrical power output that varies with wind speed so that variation occurs in voltage and frequency of the generator output as wind speed varies. For example PWM power converters may be arranged to control and switch such variable power supplies so that a resulting power input into a power network matches the nominal fixed voltage and frequency.
A method for switching power converters is described in an international patent application WO 2006/039823 entitled “Signal transmission system for activating a power semiconductor switch, and a converter equipped with a signal transmission system of this type” assigned to ABB Research Ltd. WO 2006/039823 describes a signal transmission system which serves to activate at least one power semiconductor switch (S1, S2, . . . , Sn) starting from a controller (11). At least one control signal can be transmitted from the controller (11) to at least one modulator (M1, M2, . . . , Mn) via at least one first transmission path (3). It discloses that a wireless control signal and/or a drive signal to PWM power converters can be transmitted using an optical signal path.
US2008284252, entitled “Control methods for the synchronization and phase shift of the pulse width modulation (PWM) strategy of power converters” and assigned to Converteam Tech. Ltd., describes a method of controlling a plurality of power converters 1a, 1b and 1c that can be used to interface to a supply network, ac busbar, etc. Each power converter includes a network bridge 14 operating in accordance with a pulse width modulation (PWM) strategy having the same switching period. The method includes providing the switching period of each network bridge with a different time offset relative to a time datum such that at least one unwanted harmonic in the supply network voltage is at least partially cancelled. In other words, different timing signals are sent to different converters, and these timing signals are used to locally offset each converter's clock.
A technical challenge for such implementations is that communication between a controller and each converter requires a very fast communication link with latency of only some few microseconds μs. Alternatively, the local clock of each converter must be extremely accurate over a long period to maintain sufficient accuracy down to a few microseconds, which poses both technical issues and cost issues.